Thanks, Samson, the simplest suggestion in your link of changing the schedule of Windows Defrag seems to have worked for me - thank heavens I don't have to wade through some of the other somewhat scary solutions. I've actually turned off the automatic option in Defrag (I was pretty sure I'd done that before, but perhaps not) as I feel that it's not necessary to run it all that frequently and it's a real resource hog.
Stupid of me to blame Registry Backup for the error without trying to open Task Scheduler directly first - a bad case of "Post hoc ergo propter hoc".........